This invention is concerned with optical encoders which are used to measure accurately the angular displacement of a shaft or rotating member. More particularly the invention relates to a technique for optically ascertaining the angular position of a rotating member using a combination of absolute and incremental sensing.
The need for precision measurement in a variety of devices demands an inexpensive, high resolution method for ascertaining angular position. Stepper motors, for example, as used in such diverse applications as gun turrets, paper making plants, machine tools, direct drive motors, and robotics applications, require a high precision indication of angular position. Such angular displacement information may be utilized in subsequent process steps or as a feedback indicator of system error and misalignment.
Prior art absolute encoders employ a disk having a series of abbreviated lines at various radii coupled with a multiple channel reader to directly read the position of the encoder in a binary format. While such a system is simple and inexpensive, it does not provide the accuracy required by modern high precision instrumentation.
Other prior art encoders use laser technology, such as the laser incremental encoder model X-1, manufactured by Canon. A complex series of prisms in this device is used to produce 225,000 sinusoidal waves per revolution. The 225,000 waves are each further subdivided into 80 pulses per wave to yield 18 million pulses per revolution. The principle employed in the laser rotary encoder is that of a grating disk having a series of radially oriented bars about its outer periphery. The radially oriented bars are very small and must be positioned on the disk with great precision. A grating disk having the required grating bar tolerances for the laser rotary encoder is difficult and expensive to fabricate. Furthermore, the grating disk is required to be used in conjunction with a phase plate having similarly strict manufacturing requirements.
The cost of such a high precision rotary encoder is consequently quite prohibitive. Furthermore, because of the electronic components in this apparatus, there are limitations on the operating environment in which such an encoder may be employed. The operating temperature of the encoder must remain within a temperature range of approximately 10.degree. C. to 40.degree. C. Otherwise, not only may inaccuracies be introduced into the measurements, but the encoder may be damaged or destroyed. Consequently there is a need in some low and high temperature position measurement applications to employ a device which does not require any electronic components in the position encoder head.
What is needed is an angular displacement rotary encoder having a precision and resolution which exceeds that available for encoders in its class. The cost of such an encoder should be considerably less than that associated with a complex laser rotary encoder, yet the resolution should be much greater than that obtainable with a standard direct digital read encoder.